460 
IDDINGS. 
It would require the same kind of growth to form the seg¬ 
ment of a sphere. Furthermore, the character of the growth 
which would produce a sphere would not be altered by 
changing the curvature of the spherical surface from which 
it was advancing at any moment of its growth; consequently 
it is not essentially different when it proceeds from the 
surface of a sphere whose radius is infinity—that is, from 
a plane; or even when the sign of the radius is minus— 
that is, when the surface is concave. These conclusions are 
substantiated by the study of the various forms of growth 
associated with spherulites. It is therefore logically correct 
to embrace all manner of crystalline growths which have 
the same internal structure as crystalline spherulites under 
the general designation of spher-ulitic growths or crystallizations , 
whether their cross-sections be circular, elliptical, or in 
sectors, plume-like or irregularly excentric; or whether they 
appear as straight or curving bands. 
The necessity of basing the general definition of spher- 
ulitic structures on some other character than their outward 
form is well illustrated by the microscopic spherulites of a 
typical spherulitic groundmass such as occurs in the rock 
under investigation. The absence of a spherical boundary 
to the majority of the spherulites and the frequency with 
which excentric and irregularly polygonal forms occur forces 
one to the conclusion that the fundamental characteristic of 
these typical spherulites must exist in their mode of crys¬ 
tallization. It is to be remarked that the proof of their being 
definitely crystallized bodies and not amorphous material 
under an internal strain, which might produce double refrac¬ 
tion, lies in the fact that the dark crosses which show them¬ 
selves between crossed nicols do not bear that relation to the 
shape of the excentric and polygonal spherulites which they 
should if they resulted from a contraction or irregular com¬ 
pression of globules which had assumed corresponding 
shapes. If the latter were the case they should behave like 
the dark hyperbolas of pearlite grains. The dark crosses 
retain their form with slight modifications corresponding to 
